Subminiature fuses, like other types of fuses are used to protect circuit components from damage that can be caused by excess current flowing through the circuit. Excess current is generally categorized as either an overload current or a short circuit current. Overload current is generally considered to be in the range of 135% to 500% of normal or rated current. Short circuit current may be above 500% of rated current.
To be certified by Underwriters' Laboratory, a fuse must pass certain test requirements such as a short circuit test, a low overload test and a continuity test. For the low overload test, the fuse element must open within a specified period of time at a percentage of the rated current ranging anywhere from 135-500%. For the short circuit test, the fuse element must open the circuit without rupture of the fuse body. For the continuity test, after the fuse has opened, the voltage is held for one minute during which time the fuse must not restrike.
Many conventional fuses are constructed from a fuse element and a two piece fuse housing comprising a cap and a base. During a short circuit condition, pressure inside the housing increases. Due to the small physical size of the subminiature fuse and hence the short arc clearing gap, that is the short distance between the terminals carrying the fuse element, the housing for such a fuse is subject to catastrophic failure problems that are not normally inherent in a physically larger fuse. There is a risk that the fuse housing will blow apart or rupture.
In the two piece housing design, this rupture normally occurs at the seal between the cap and the base. If the housing ruptures, this would not only expose a live arc but would also prolong that arc thereby potentially causing damage to circuit components downstream of the fuse due to the additional time required to fully interrupt the circuit. Once the housing begins to leak, the pressure in the housing begins to decrease. This causes the interruption time to increase.
Those skilled in the art know that when the fuse element is subjected to short circuit current, the fuse element heats up until it reaches the melting point of the fuse element conductor. The rate of the heat build up is, among other things, a function of the magnitude of the excess current. Once the temperature of the conductor reaches its melting point, the conductor material is rapidly vaporized mixing vaporized metal atoms with the gas or air medium surrounding the conductor. An arc is formed in the gas mixture or plasma which acts as a conducting path for the arc. The increased temperature of the arc plasma also increases the pressure in the fuse housing. If the arc plasma becomes dense, the travel of the charged particles in the plasma is restricted. Decreased mobility of the charged particles increases the resistance of the gap, thereby acting to extinguish the arc which is necessary for the proper functioning of the fuse in interrupting current to the circuit. Thus it is seen that the increase in pressure in the fuse housing is beneficial in extinguishing the arc and interrupting current to the circuit. Prior art fuses have tended to rupture during increases in pressure.
An attempt to overcome this problem is illustrated in U.S. Ser. Nos. 715,799 and 852,605. These patent applications disclose a subminiature fuse comprising two terminals, a fusible element between the terminals supported on a ceramic substrate enclosed in a ceramic coating and surrounded by a unitary housing. While this fuse has been found to work well, a point is reached in the ampere rating where the fuse can no longer contain the pressures resulting from a short circuit. This is especially true for voltages of 250 volts or higher.
Therefore, it is an object of present invention to increase the short circuit performance of the subminiature fuse and thereby increase the maximum ampere rating attainable.